Re: Easing Into CNC -- Ball Screws?

Paul:

I don't know why I'm attempting to answer this. From you background I'm
convinced you have a better handle on the situtation than I. But it's a
rainy day. :-)

I moved to ball screws right off for a couple of reasons. My acme lead
screws were worn and beat-up. I adjusted them and got 3 or 4 thousands
backlash but I'm convinced they would wear quickly and soon be worse
than this. The other reason was to avoid building bearing plates twice.
I was afraid there would be variations between my existing acmes and any
retrofit lead screws. If they did differ I'd have to modify or re-build
my bearing/motor mounts for the new dimensions. So, I purchased a
retrofit kit from Rockford and built my new bearing block/motor mounts
to these.

If you build motor mounts, etc for your existing acme lead screws you
may well have to re-build or modify them when you move to ball screws.

Other than this I suspect acmes will work OK. You mention back-lash and
the possibility of "slaming" through this backlash due to climb cutting
or similar are the problems. Also the acmes will give more friction than
ball screws as you say. You can calculate an estimate of added torque
due to this friction. But without doing this, I'd suspect you could size
the motors for ball screws and only lose some speed if used with acmes.
If you use a timing belt drive you could certainly trade torque for
speed by changing out the sprockets.

I'd suggest you consider driver kits or purchased drivers rather than
design and build your own. I purchased stepper driver kits from
Camtronics and have been very pleased. Even if I could have designed
these the time I would have saved would be significant and the cost
savings minimal. Unless you just want to tackle this design. :-)

As you say, just load up linux and EMC. I did this, but it took a month
or so of hard work. But you probably have more linux experience than I,
most do. :-) I do like EMC.

Hope this is of help.

Hugh Currin
Klamath Falls, OR

On 12 Nov, Paul R. Hvidston wrote:
> I'd like to solicit some words of wisdom from the group.
>
> I decided it was time to join the group since I'm considering moving towards
> making my RF-31 dual-use. Here's the relevant background: As a consulting
> engineer, I've done a lot of automation system design and implementation,
> most recently (several years back) as a certified system integrator for
> Cimetrix -- awesome software, poor company (IMO). All of my work was in the
> area of micron-precision equipment. Anyway, just to keep playing with
> computer control, I want to incrementally (cheaply) add CNC to my RF-31.
> I've got an industrial PC that I'll be loading LinuxCNC (NIST EMC), and I
> can put together the stepper controllers and power supplies, so that's not a
> problem. However, even after searching through the message archives, I'm
> still wondering if I can "get away" with retaining the stock lead-screws and
> nuts for the near term. I understand some of the issues of backlash
> (climb-milling and accuracy), but given that I can minimize backlash with
> bearing pre-load and a well-adjusted nut (:-o), what else is there to worry
> about if using acme screws instead of ball screws? Friction? Would I need
> much bigger steppers to offset friction (eating up any savings in lead
> screws)? Your advice cheerfully accepted.
>